Casting process involving cooling of a shell mold prior to casting metal therein

ABSTRACT

Method of precision casting of high temperature, nickel-base alloys in ceramic shell molds wherein, immediately prior to casting, a preheated shell mold is permitted to cool to provide a condition wherein there are substantial temperature differentials throughout the shell mold. Substantially enhanced characteristics are obtained in the resultant casting.

United States Patent lnventor John llockin I Palatine, Ill.

Appl. No. 684,916

Filed- Nov. 22, I967 Patented Jan. 5, 1971 Assignee Martin Metals Company Wheeling, 111., a corporation of Delaware, by mesne assignments CASTING PROCESS INVOLVING COOLING OF A SHELL MOLD PRIOR TO CASTING METAL THEREIN 4 Claims, 1 Drawing Fig.

us. (I 164/65, 164/121, 164/122 Int. Cl B22d 27/16 164/121,

Field of Search l22,125,65,6l;75/l7l [56] References Cited UNITED STATES PATENTS 3,153,824 10/1964 Holmes 164/65 3,200,455 8/1965 Operhall et al. 164/121 3,274,652 9/1966 Banks 164/125 3,279,006 10/1966 Schwartz et al 164/65 Primary Examiner-Andrew R. Juhasz Assistant Examiner-V. K. Rising Attorneys-John A. Crowley, Jr. and Francis J. Mulligan, Jr.

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Va cuu M EuvuRoNMiINT J Q Hamaw lilv'lvfa flrra R N Y CASTING PROCESS INVOLVING COOLING OF A SHELL MOLD PRIOR TO CASTING METAL TI'IEREIN The present invention is concerned with casting of alloys and, more particularly, with the casting of nick'el-base, chromium-containing super alloys in ceramic shell molds. When parts, subjectedin'use tohigh stress at high temperatures, must be made of nickel ba'se alloys containing chromium and elements adapted to provide hardening by the mechanisms of solid solution strengthening, carbide-hardening and gamma prime precipitationjand su'ch alloys are not adapted to be forgedto shape, it is most often convenient to cast such alloys substantially to shape by preci'sioncasting techniques using ceramic-shell molds. Inthis way. expensive grinding and machining operations on least parts are minimized. In general, a shell mold is a mold havinga coherent ceramic body adapted to contain'molten'metal within walls having thicknessesbetween aboutone-eighth to about one-halfinch. Shell moldsareusuallyinade by repeatedly dipping a fugitive model ina ceramic slurry containing a binder-material, stuccoingthe dipped model with particulaterefractory between dips and, 'aftersufficient thickness, cg! aboutthree-eighthsinch, of dipped coatings has been obtained, causing the fugitive model to be "removed" from the built-up mold. Asan ,example, one can producea shell mold coating awax model of'a turbine blade with refractory such as zircon -or alumina. As indicated above; the coating is provided byfi'rst dipping the model into "a' slurry of very finely-divided refractory, removing the slurry coated model from "the dipping bath and stuccoing the slurry coated model with particulate refractory having a' particle size substantially in excess of the particle size'of the refractory in the' slurry.. This dipping and stuccoing process is repeated until the required thickness of approximately three-eighths inch is";obtain ed. After the coating onthe model is dry .and a temporary bond is obtained, the model can be removed by dissolution, melting, burning or temperatures, a phenomenon termed creep occurs. Creep is generally considered to be an irreversible deformation, the magnitude of which deformation is dependent in part upon time and in part upon temperature and stress. Eventually, creep will cause failure either by causing the metal part to extend beyond limits of tolerance or, in drastic cases, to fracture. Generally speaking, the higher the temperature of use, the more severe is the creep problem assuminga constant applied stress. Conversely, generally speaking, the greater the stress, the more severe is the creep problem assuming a constant temperature. The resistance of a metal to creep at various temperatures is not usually uniformly high or low. It is not only possible but usual for a metal part to exhibit good creep resistance at l,800 F. and relatively poor creep reslstanceat l,400 F. Conversely, a part made of identical metal chemistry but somewhat difl'erent prior thermal history may have relatively poor creep resistance at l,800 F. and relatively good creep resistance at l,800 F. and relatively good creep resistance at -l ,400 F,

It is known-that many factors are involved in the actual performance of metal structures under creep inducing conditions.

The present invention is concerned with one of these factors,

towit: the solidification rate and the rate of cooling of precision cast metal, cast into ceramic shell molds such as described hereinbefore. The solidification rate and the rate of cooling are primarily affected by the pouring temperature of the metal and the initial mold temperature -at'the start of casting. It is known that with many nickel-base alloys of the kind in question, if relatively slow cooling rates are used by providing an initial high mold temperature, good 1-,400 F. creep the like.yThe shell mold is then matured and provided with a ceramic bond by firing at an elevated temperature; for example, at about 1,600 to 2,000 F. By now, such 'shell molds are common items used in the precision casting industry although, in view of their peculiar nature, they are not generally eonsideredto be articles of commerce; Particular ways of manufacturing shell molds are set forth-and described in the literature particularly in, for example,-' U.S.;Pat. Nos. 2,932,864;

3,132,388 and 2,961,75l; 1-:

In physical form, shell molds','the manufacture of which is described generally; in the preceding paragraph, include not only a cavity in the shape'of the article to be'mold'e'd but also connecting cavities in the shapesof feeders, 'risers,.spru es, pouring basins and the like suchthat all of ,the features required by proper casting technique are incorporated in the ceramic shell mold. Quite often, but not necessarily, the shell mold is employed in vacuum casting. ltis well known that the normal atmosphere can contaminate inost metals when the metal is in the molten state. With common metals'such'as steel, it is quite usual to at least partially protect the molten metal from atmospheric contamination by melting and pour- .ing under a slag cover. When alloys contain eyen relatively small amounts of elements, such as titanium, aluminum, zirconium, boron and the like, it is highly'advantageous to melt and cast such'alloys in vacuum. Again, bynow, vacuum melting and casting of alloys sensitive to atmospheric contamination is well known in the art. Further, it is, of course, well known in the art to vacuum cast 'such alloys into ceramic shell molds of the type described hereinbefore. g

Certainnickel-base alloys containing, chromium (primarily for'oxidation resistance) and other elements designed to provide'hardening by means of solid solution strengthening, carbide strengthening and gamma pri'rneprecipitation are especially adapted to becast in vacuum. Many of such alloys are net forgeable except in an extremely limitedway and are par-- I jected to stress below their-yield point especiallyat elevated rupture properties and ductility can be provided. Under such casting conditions the creep resistance'properties at l,800 F of the cast metal are relatively low. Conversely, if a low mold temperature is used to'provide extremely rapid cooling, ex-

tremely good creep properties can be provided for use at l,800 F. but only at a sacrifice of properties at l ,400 F., such as ductility. If it were possible to'design turbine engines to operate uniformly at a single high temperature the aforedescribedcasting vargaries would provide no difficulty. in practice, however, the high temperature section of a-jet engine does not operate at a-single temperature. The temperature of a single turbine blade in the hot section-of a jet engine can varyin different spots across its cross section and in different spots along its length. It is not impossible that a single practical operating turbine blade would involve in different areas thereof temperatures ranging from 1,400 to l,800 F. at any given moment of operation. Furthermore, during starts and stops the temperatures in the hot section of a turbine engine can vary over the range of from' ambient atmospheric temperatures to up to 2,000 F. For these reasons it is important to provide cast structures having optimum properties over a reasonably wide range of elevated temperatures. The present invention is concerned with vthe solution of this problem which, as far as I am aware, has not heretofore been solved in a satisfactory manner on a commercial scale.

It is an object of the present invention to provide a novel process for the production of structures subjected in use to stress at a range of elevated temperatures.

It is another object of the present invention to provide structures for use under stress over a range of elevated temperatures.

Other objects and advantages of the present invention will become apparent from the following description taken in conjunction with the drawing which depicts the process of the invention in schematic form.

Generally speaking, thepresent invention contemplates the process of casting and the'product produced thereby wherein an alloy (metal), selected from the group of certain nickelbase alloys specified hereinafter,-in molten form at a tempera ture of about 200 F. above the freezing point thereof is poured into a ceramic shell mold characterized just prior to the-start of pouring by-having different temperatures in different areas in and on the'mold with the spread-of the temperatures being in the range of about l to 350 in TABLE III Fahrenheit units. and the poured metal is thereafter allowed to cool and solidify in said shell mold. The maximum and gggi: minimum of the aforementioned temperatures both are (percent between the limits of about 400 to about l200 in Fahrenheit 5 i ht) units below the freezing point of the metal. Advantageously m n the maximum of said temperature spread is about 800 in Chrommm 8-18 Fahrenheit units below the freezing point of the metal and the gobalt 0-20 ungsten 0-13. 5 minimum IS about 1,100 In Fahrenheit units below the freez- Molybdenum ing point of the metal. For purposes of this specification and A1uminum claims. the freezing point of an alloy (metal) shall be con- Tit i 0-6 sidered to be the liquidus of the particular alloy composition Aluminum plus titanium 6-11 employed. C rbon 0. 02-0. 3 The nickel'base alloys particularly adapted to be employed P 02 in the process of the present invention are nickel-base. gg gi chromium-containing alloys having in the solid state a gamma 31 matrix, a gamma prime precipitate dispersed throughout the '::III:IIII: matrix upon initial solidification of the alloy from the molten (j l bi 0-6 state. elements in solid solution to strengthen said gamma Nickel 1 Balance matrix and carbide particles dispersed throughout the alloy l Together with impurities and incidental elements which do not adbody. Generally speaking, such alloys will contain at least verse/1y meet the utility of the alloys about 5 percent of an element selected from the group consisting of titanium and aluminum or both, at least 2 percent of an element selected from the group of molybdenum, tungsten. columbium, or tantalum. at least about 0.02 percent carbon When lloy within the range of composition set forth in and. advantageously. small amounts of boron and z co um. table [II are cooled from the molten state, carbide phases are for example up to about 0. 2 perce t f bOfOn n p to about initially formed in the solid metal followed by gamma prime 0.3 percent of zirconium. Advantageously, such alloys can re ipitate. Many of the alloys adapted to be used in the contain up to about 2 perc n Cob l and may HIB H process of the present invention are not amenable to be soluamounts of iron, manganese, silicon and other incidental elenon-treated and aged to redistribute and/or reform carbide ments which do not affect the basic characteristics of the aland gamma prime phases. loys. As is well known to alloy developers, the composition of In carrying out the process of the present invention one of such alloys will be balanced so as to avoid the formation of the easiest ways to achieve the required temperature profile of embrittling phases during exposure to elevated temperatures. the shell mold, as described hereinbefore, is to initially heat A typical alloy which is particularly adapted to be treated by the mold to a temperature of l.900 F. and then cool the mold, the process of the present invention is that alloy known in advantageously in vacuum, for about2 to about l5 minutes or commerce as B-l900 which has a nominal composition as set more before casting metal therein. If the cooling is accomforth in table I. plished primarily by radiation, it is thought that the inner walls of the mold should cool slower than the outer walls because the heat sinks to which the inner walls will be radiating (each TAB I other) will be much higher in temperature than the sinks to Perqent which the outer walls radiate. The same should essentially be Element: by Welght true if the shell mold is cooled in air provided one avoids cool- Chromiurn 0 ing the mold by pumping cooling air through the mold cavity. M yb 0 In practice, however, the temperature profile of the cooled Tltamum 8 shell mold does not seem to be simply that the inner, metalgfigg 0. 1 contacting walls of the shell mold are 200 to 300 in 0 Fahrenheit units hotter than the outer walls of the mold with Cobalt 10 0 intermediate temperatures therebetween. Apparently the C b O. 10 cooling process is more complex than what would be thought Tantalum 0 to be the case. The complexity of the cooling process in- Nickel Balance troduces temperature variations throughout the whole mold whereby certain areas are hotter by up to about 300 in Fahrenheit units than the coolest areas of the mold. When The nominal compositions of other alloys which can be emmetal such as B-l900 is cast using a pouring temperature of ployed in the process of the present invention are set forth in about 200 F. above the freezing point into the mold cooled table II. from l.900 F. to provide the aforementioned thermal TABLE II Alloy 01' Mo Cb Ti Al B Zr Co C Ni W Ta Fe V INCONEL alloy 713C 2. 0 8 6.1 0. 012 0.10 12 Balance MAR M 2 alloy 200- 1 2 5 0.015 0.05 10 0. d IN 100 100 3 47 5.5 0.014 0.06 15 0. LIAR M alloy246 00 2.5 15 5.5 0.015 0.05 10 0 l INCONEL is a trademark of The International Nickel Company Inc. 1 MAR M is a trademark of Martin Marietta Corporation. The alloys sold under this trademark were formerly sold under the trademark S-M.

From the foregoing tables and description it is to be observed that alloys having compositions within the ranges set forth in table lll are adapted to be used in the present invention.

gradient throughout the mold. one obtains an excellent combination of stress rupture and creep rupture life at 1,800 and l,400 F. Advantageously. when cooling by radiation in a 'vacuum from an initial mold temperature at l.900 F.. the

cooling period is about 3 to about minutes. for example about 5 to about 9 minutes, to optimize all properties. it is to be recognized that the present invention contemplates casting metal into a shell mold having the defined temperature dif- Table IV shows that. by using the process of the present invention. one can provide structures made from alloy B-l900 having an excellent combination of high temperature mechanical characteristics, the measured characteristics ferentials no matter how such a condition is created. The 5 being essentially equal if not superior to the optimum stresssequence of practical steps of the process of the present inven tion involving cooling for about 2 to about minutes is depicted in schematic fon'n on the drawing.

In order to give those skilled in the art a greater appreciation of the advantages of the invention, the following examples are given:

Example I rupture characteristics obtainable at either 1,400 F. or l,800 F. using prior molding techniques A or B.

Example ll An alloy having the chemistry of that alloy identified in table ll as'MAR M alloy 246 was melted and cast in exactly the same fashion as was described in Example 1 except that the mold was allowed to cool for about [0 minutes. Table V paral- An corresponding to h h i Set f th i table 1 15 lels table IV in showing the advantageous results obtainable which is known in the industry as 8-1900 was melted under vacuum and held at a temperature of approximately 200 in Fahrenheit units above the freezing point as measured by an uncorrected optical pyrometer. At the same time a shell mold, as described hereinbefore, containing a plurality of cavities conforming to the shape of stress-rupture specimens was heated to 1,900 F. and allowed to cool for about seven with the process of the present invention.

Lifetd-rupture at 1,800 F. under a stress of 32,000 p.s.i. (hours) 27 50 53 Life-to upture at 1,400" F. under a stress of 100,000 p.s.i. (hours) 79 141 Equally impressive results are obtained with other alloy maintained at about 1,6000 and (C) as cast into a mold 30 compositions of similar nature such as those set forth in table under the conditions described in example I.

Nom.p.s.l.=pounds per square inch.

II and within the compositional limits set forth in table lll. By means of the present invention one can produce structures having a remarkable combination of high temperature characteristics making the structures particularly applicable for use in the hot sections of gas turbine engines. When the present invention is used in conjunction with state of the art techniques relating to mold preparation, melting and casting, one can expect to achieve overall superior properties in alloy structures which are usable at temperatures from room temperature to the highest temperature limit of the alloy in question. Structures producible in accordance with the present invention include gas turbine blades, vanes, integrally cast turbine wheels, and other applications of high temperature alloys.

Example lll As a final example of the utility and unobviousness of the present invention a relatively newly developed high temperature nickel-base chromium-containing alloy was cast into test bars using a ceramic shell mold cooled-back from l.900 F.

for various amounts of time.

TABLE VI in the range of about l,200 F. to 2.000 F. and comprising, as a solid. a matrix gamma phase, a gamma prime phase and a carbide phase, comprising the steps of:

a. melting said alloy under vacuum;

b. superheating said alloy to about 200 in Fahrenheit units above the freezing point thereof while maintaining said vacuum to place said alloy in condition for pouring into a 1 Stress rupture specimens subjected to a load of 27.5 K s.i. (27,500 p.s.l.) at 1,800" F.

Table VI shows that the reduction-of-area of the relatively newly developed alloy in hot tensile test at l,600 F. is greatly improved, e.g. over I00 percent improvement, without detriment to other characteristics significant from an engineering standpoint. At least one major jet engine manufacturer considers the reduction-of-area during hot tensile testing to be a highly significant factor in determining the suitability of an alloy or alloy structure for hot stage jet engine service. Table VI shows that hot tensile reduction-of-area can be raised from l0.5 percent up to 22.7 percent and even higher for this particular alloy by means of the present invention without significantly varying the stress rupture life and accompanying reduction-of-area at 1,800" F. under a load of 27.5 k.s.i. from the average values of 24.8 hours and 14.2 percent, respectively, and without significantly varying the Ultimate Tensile Strength (U.T.S.) and 0.2 percent offset Yield Strength (Y.S.) at l,600 F. from the average values of 102.6 k.s.i. and 77.6 k.s.i., respectively.

While the present invention has been described in conjunction with advantageous embodimepts tl ose s killed in it; art will recognize that modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention.

lclaim:

l. A process of casting nickel-base high temperature alloy, said alloy being adapted to be used under load at temperatures mold;

c. initially preheating a refractory shell mold to about 1.900" F. in vacuum; causing said initially preheated refractory shell mold to cool by radiation in said vacuum from about 1.900 F. for about 2 to about 15 minutes to provide a refractory shell mold, characterized by having a diversity of temperatures at diverse locations therein and thereon with a spread of about l00 to 350 in Fahrenheit units between the maximum and the minimum of said temperatures, said maximum and minimum of said temperatures being between about 400 and l,200 in Fahrenheit units below the freezing point of said alloy;

. immediately after said cooling, casting said superheated alloy into said cooled shell mold while still maintaining vacuum thereon; and

. thereafter maintaining said vacuum conditions until said alloy has frozen in said shell mold whereby said alloy thus cast exhibits maximized mechanical characteristics in both the lower and the higher portions of said temperature range of about l,200 F. to 2,000 F.

2. A process as in claim 1 wherein the refractory shell mold is made of zircon or alumina as is made by the lost wax process.

3. A process as in claim I wherein the refractory shell mold is cooled from about 3 to about 10 minutes.

4. A process as in claim 1 wherein the refractory shell mold encloses a volume in the shape of a gas turbine blade.

Patent: No.

Inventor(s) 3,552,479 Dated January 5, 1971 John Hockin It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, lines 18-19, "1800F. and relatively good creep resistance at" was incorrectly restated and should be cancelled;

Column 6, in Table V last column in table under "C" "Example I" should be Example II Column 7, lines 44-46, "Such modifications and variations may be resorted to without departing from the spirit and scope of the invention." was incorrectly stated and partly repeated and should be cancelled.

(SEAL) Attest:

WILLIAM E. SCHUYLER,

EDWARD I'I.FLETCHER,JR.

Commissioner of Patenl Attesting Officer 

2. A process as in claim 1 wherein the refractory shell mold is made of zircon or alumina as is made by the lost wax process.
 3. A process as in claim 1 wherein the refractory shell mold is cooled from about 3 to about 10 minutes.
 4. A process as in claim 1 wherein the refractory shell mold encloses a volume in the shape of a gas turbine blade. 